There are two motions that take place when a vehicle turns. The first motion is a turning motion called yaw motion. Yaw motion takes place as the vehicle body spins around an imaginary axis that is vertical to the ground. The second motion is a lateral sliding motion called side-slip motion. Side-slip motion occurs in the same direction as the turn or in the opposite direction depending on the speed of the vehicle.
A desired yaw rate and side-slip velocity are determined based on the speed of a vehicle and the position of the steering wheel. The desired values correspond to the expected yaw rate and side-slip velocity when a vehicle is traveling on a dry and clean surface. When the actual yaw rate and/or side-slip velocity significantly surpasses the desired values, the driver feels a loss of control of the vehicle.
The actual yaw rate and side-slip velocity of the vehicle are compared to the desired values. Corrective action is taken when the desired values are surpassed by a predetermined threshold. When a significant discrepancy exists between the desired yaw rate and the actual yaw rate and/or the desired side-slip velocity and the actual side-slip velocity of the vehicle, it is likely the road conditions necessitate vehicle stability enhancement.
Current methods of vehicle stability enhancement include using yaw rate feedback and side-slip acceleration feedback control signals. The yaw rate of a vehicle can be measured using a commercially available yaw rate sensor. The side-slip velocity of a vehicle can be measured using side-slip velocity sensors, which are very expensive. Instead of using a sensor, side-slip acceleration can be estimated based on the lateral acceleration, yaw rate, and speed of a vehicle. Ideally, the side-slip velocity of a vehicle can be obtained by integrating the side-slip acceleration. However, since sensor bias exists in yaw rate sensors and lateral accelerometers, the integration tends to drift due to the unwanted bias signal being integrated.
In one conventional approach, a vehicle stability enhancement system uses yaw rate feedback and side-slip angle feedback (which can be derived from side-slip velocity) to create a corrective yaw moment to correct a dynamic behavior of a vehicle. The estimation of side-slip velocity is implemented using a dynamic observer that captures the estimated state of dynamics of the vehicle. However, the estimation is based on a vehicle's cornering compliances, which are variable vehicle parameters. The cornering compliances vary over a wide range and depend on the type of surface that the vehicle is operating on. Therefore, the estimate is not as accurate as desired.